A method and an apparatus for the manufacture of a compound pipe of the species is known from EP 0 108 598 B1, in which the external tube is acted upon from inside by compressed gas, whereby it is forced into annular mold recesses of the half shells. Via the wall of a temperature-regulating bell the internal tube is pressed against the corrugations of the external tube and welded together with it. For the production of a pipe socket within the continuous pipe, the external tube is pressed into a cylindrical, smooth socket recess in the half shells. The internal tube is molded in parallel at a corresponding distance. Then--the internal tube and the external tube still being thermally plastic--one of the two tubes is punched through and a mechanical pressure is exercised from inside on the internal tube, so that the latter is pressed against the external tube and welded together with the latter. In terms of construction such an apparatus is of considerable expense. Moreover, practice has shown that a reliable welding connection and a true-to-size production of a pipe socket is not possible.
A method for the manufacture of compound pipes is known from PCT WO 90/14208, in which an external tube is pressed into annular mold recesses of the half shells without this being specified in detail. By means of a partial vacuum exercised on its inside an internal tube is held on a temperature-regulating bell and is welded together with the external tube in known manner. For the production of sockets, the internal tube is acted upon by compressed gas from a gas duct located downstream of an internal nozzle and is pressed all-over against the external tube and welded together with the latter. A defined connection between the internal and the external tube is not assured.
EP 0 385 465 A2 teaches a method for the manufacture of compound pipes with an integral socket, a normal corrugated compound pipe being widened at one end to form a socket and the corrugation at the other end being reduced to form a spigot.
A method and an apparatus for the manufacture of a compound pipe with an integral socket is known from PCT WO 88/05377, in which an external tube is sucked by a partial vacuum into the annular recesses of the half shells and in which, section by section, an internal tube is welded together with the corrugated external tube. To form a pipe socket, a socket recess, into which the external tube is sucked, is provided in at least one pair of half shells. When the external tube has been completely molded into the pipe socket, then the internal tube is pressed by compressed air against the external tube and is welded together with the latter. To this end a temperature-regulating bell is provided with an annular gas duct at a considerable distance behind an internal nozzle extrusion-molding the internal tube. This gas duct is acted upon by compressed gas, when the entire pipe socket molded from the external tube and the to-be-deformed portion of the internal tube are located on the temperature-regulating bell. It is not ensured by this construction that the welding together of the internal tube and the external tube in the vicinity of the pipe socket is reliably performed.